1. Field of the Invention
The present invention relates generally to electric resistance vacuum heat treating furnaces; and more particularly to improvements in a high temperature electric resistance vacuum furnace suitable for heat treating processes, such as brazing, tempering, degassing, sintering, and hardening, in which the hot zone is heated by radiant energy and cooled by recirculated fluid.
2. Description of the Prior Art
Electric vacuum heat treating furnaces typically consist of a cylindrical water-cooled vessel containing heating elements forming a hot zone for receiving, a workload to be heat treated. An example of such a furnace is disclosed in U.S. Pat. No. 3,438,618 to Seelandt in which a cylindrical vessel contains a retort of separate upper and lower water-cooled, U-shaped shells with end walls movable into side-by-side relationship to form a box-like chamber. Radiant heating elements line each shell in transverse planes axially spaced along the length of the chamber. Additional elements in flat grids line both end walls. Multiple nested layers of radiant heat-reflecting shields reflect some of the radiation from the elements back into a hot zone work space. The furnace is evacuated by an oil diffusion pump to provide a non-oxidizing atmosphere during the heat treating process. A quenching fluid of inert gas may be injected into the chamber after the heating phase of the process is completed and recirculated through a heat exchanger for rapid cooling. U.S. Pat. No. 4,559,631 to Moller teaches annular banks of heating elements in planes axially spaced in the furnace. The banks of elements may be differentially located and/or energized to establish front-to-rear temperature trim zones. U.S. Pat. No. 3,185,460 to Mescher et al. and U.S. Pat. No. 3,257,492 to Westeren disclose elongate heating elements coaxially mounted in the furnace and mutually spaced from each other.
The heating elements are usually fabricated in flat bars of graphite or refractory metals such as commercially pure molybdenum in rectangular cross section as shown in Moller, supra. Seelandt, supra, proposed another element design which is elliptical in cross-section and of substantial thickness. The convex surfaces of the element face inwardly toward the middle of the chamber and outwardly toward the heat shields.
While prior art electric vacuum furnaces as above-described are satisfactory for many heat treating processes, they are lacking in certain design features which significantly improve efficiency in the process. Heating elements of thin rectangular or elliptical cross sections are prone to sag under high temperatures between spaced apart supports because of low section modulus. The rectangular and elliptical elements also inherently lack even distribution of emitted radiant energy from all surfaces for achieving the precision demanded. The radiant energy is emitted in opposite directions substantially perpendicular to the flat sides, consequently, energy directed toward a heat shield is merely reflected back to the element instead of onto the workload. Elements with elliptical or similarly curved surfaces direct only a portion of the radiant energy emitted toward the heat shield for reflection onto the workload. The above-described heating element designs choke a significant percentage of the emitted radiant energy which reduces the effective surface area and results in higher element temperatures causing creep, sagging, and non-uniform heating. Hence, the temperature of the workload will not be of optimal uniformity and a relatively long heat treating cycle time is required. When quenching fluid is recirculated in the furnace through a heat exchanger at completion of the heat treating phase, the extremely hot fluid returning to the heat exchanger may heat seals and other components therein beyond their design limits causing permanent damage and leakage.